Until recently, mast cells and basophils were considered to be rather obscure cells of the immune system whose main role is to induce allergy. However, increasing evidence over the last few years has illuminated their critical role in the initiation and regulation of the immune system. It has also become evident that improper functioning of these cells is central to the pathogenesis of chronic inflammatory and autoimmune diseases such as multiple sclerosis and rheumatoid arthritis, and even cancer. Hence, understanding the ontogeny and development of these cells has become imperative. Considering the fact that mast cells and basophils are similar in several aspects including the relatively unique expression of the high affinity IgE receptor and share common modes of activation, degranulation and mediator release, it is not surprising that they have parallel developmental pathways as well. It has been reported that mast cells and basophils arise from a common hematopoietic precursor - the basophil-mast cell precursor (BMCP). However, the molecular mechanisms directing lineage choice between mast cell vs. basophil are unclear. The granulocyte-related transcription factor CCAAT/enhancer-binding protein 1 (C/EBP1) is reported to steer the common precursor towards the basophil lineage. We have recently demonstrated that the transcription factor Ikaros regulates mast cell IL-4 gene expression. Furthermore, preliminary data from our laboratory suggests that Ikaros is involved in mast cell development and without Ikaros, cells default to the basophil lineage. In this proposal we hypothesize that the transcription factor Ikaros drives the lineage choice of the BMCP towards mast cell lineage. As set forth in these aims, we will first determine whether Ikaros regulates lineage choice towards mast cells at the expense of basophils, in a cell intrinsic manner, using Ikaros-null mice. Secondly, we will determine whether Ikaros suppresses basophil lineage promoting transcription factors such as C/EBP1. Finally, we will assess the in vivo relevance of loss of Ikaros in mast vs. basophil development by analyzing basophil specific responses in Ikaros-null mice. Results obtained from these experiments will further our understanding of lineage choice decisions between mast cells and basophils and elucidate potential therapeutic targets for inflammatory and autoimmune disorders. PUBLIC HEALTH RELEVANCE: All cells of the hematopoietic lineage, including red blood cells, platelets and white blood cells, can arise from the same single precursor stem cell. This stem cell must go through unique development steps (lineage choices) that involve the activation or suppression of specific genes in order to develop correctly into the various cell types of our blood system. Our studies focus on the development of mast cells and basophils, highly related cells that not only are the major cause of allergic symptoms, but are also critical for some protective functions of our immune system: Here we propose to study the mechanisms by which a common precursor cell chooses to become a mast cell or basophil.